Recently, the amount of generated information rapidly increases because of the extension of data centers and others as a result of the evolution of the Internet environment, the infiltration of cloud computing and others. It can be safely said that a magnetic storage apparatus such as a magnetic disk unit (HDD) the recording density of which is the highest and which is excellent in a bit cost plays a leading role of a storage in the age of big data. Therefore, the great increase of the capacity of the magnetic storage apparatus and the densification that supports it are essential.
High densification is based upon a scaling rule and it is essential to reduce the track pitch of a magnetic head, spacing between the head and a medium, a crystal grain of the medium and others. However, when the crystal grain of the medium is reduced, anisotropic energy which tries to keep a magnetized state is reduced and the recorded magnetized state is apt to be disturbed by thermal disturbance. This phenomenon is called superparamagnetic effect. Therefore, as described in Y. Shiroishi, et al, “Future Options for HDD Storage”, IEEE Trans. Magn. Vol. 45, no. 10, pp 3816-3822 (2009), it is said that in a generation of approximately 1 Tb/in2, the mere extension of the current technique will hit on a wall that shows a limit of practicality. This is called superparamagnetic limit or trilemma.
For high densification, it is the maximum subject to develop technique that surpasses this, and in the meantime, proposed in JP-A No. 1995-244801 is a spin heating recording method of supplying a high-frequency field that meets a magnetic resonance condition to a magnetic recording medium by making a high frequency source provided outside follow the motion of a magnetic head and also enabling writing to the high-coercive force medium with the low magnetic field without practically raising the temperature of the medium utilizing the nature that the spin of the magnetic recording medium absorbs the energy of the high-frequency field and the coercive force is effectively reduced.
In such a spin heating recording method, since the high-frequency field in which power is concentrated in a frequency range in which the magnetic resonance condition is met is applied, only spin is selectively excited, that is, spin heating is enabled without too much exciting a degree of internal freedom of the plasma oscillation and the lattice vibration of a conduction electron except spin, and the rise of the temperature of the whole medium such as a temperature in general heating is not caused. The above-mentioned method of applying the high-frequency field in a microwave band to the magnetic recording medium and performing magnetic recording utilizing the magnetic resonance phenomenon is called microwave assisted magnetic recording (MAMR)(the above-mentioned “Future Options for HDD Storage”).
In addition, for another method of utilizing a high-frequency field, disclosed in JP-A No. 2008-34004 is a thermally assisted magnetic recording method of applying a writing magnetic field to at least a part of the following part and writing to a magnetic recording medium after heating the following part by applying a high-frequency field in a part of a magnetic recording layer or in the vicinity of the part and generating eddy current and temporarily reducing the coercive force of the part. Further, as a method of simply radiating energy in the thermally assisted magnetic recording method, disclosed in JP-A No. 2005-285242 is a spin torque type or a spin resonance type spin microwave generator provided with at least two magnetic thin films as a high-frequency oscillator.
Recently, a practical spin torque oscillator (STO) having microstructure utilizing a field generation layer (FGL) that generates a high-frequency field with high-speed spin by spin torque is proposed in X. Zhu and J.-G. Zhu, “Bias-field-free microwave oscillator driven by perpendicularly polarized spin current”, IEEE Trans. Magn., vol. 42, pp. 2670-2672, 2006 and U.S. Pat. No. 7,616,412B2. Next, disclosed in J-G. Zhu, X. Zhu, and Y. Tang, “Microwave Assisted Magnetic Recording”, IEEE trans. Magn., Vol 44, no. 1, pp 125-131(2008) is a microwave assisted recording method for realizing densification of arranging a high-frequency oscillator (STO) having the same type structure next to a main magnetic pole of a vertical head, exciting the precession of medium magnetization with a high-frequency field in a microwave band from STO and magnetically recording information in a magnetic recording medium provided with large magnetic anisotropy, reducing a switching field.
Further, disclosed in JP Patent No. 4255869 is a method of more efficiently inducing the reversal of magnetization by making a high-frequency field oscillator generate a high-frequency field having spin in the same direction as a direction of the precession of the magnetization of a magnetic recording medium for which the reversal of magnetization is desired according to the polarity of a recording magnetic field. Hereby, research and development for realizing the microwave assisted recording method are recently accelerated rapidly, and disclosed in JP-A No. 2011-113621 is for the practical application of the method, a head drive controller that supplies a high-frequency oscillator driving signal at a higher level than a steady level by fixed effective time according to input to a write gate in a state in which a recording magnetic field is applied so as to secure the reliability of a high-frequency oscillator required for microwave assisted recording and securely maintain its oscillation.